CN104350168B - Electric-resistance-welded steel pipe - Google Patents

Abstract

Electric-resistance-welded steel pipe that the present invention is suitable for the spool to deep-sea laying, that there is enough intensity, low-temperature flexibility and low yielding ratio, it is characterized in that, the composition of mother metal is in terms of quality %, containing C:0.05～0.10%, Mn:1.00～1.60%, Nb:0.005% less than 0.035%, Ceq is 0.23～0.38, and the metal structure of mother metal comprises 3～the martensite of 13% and balance iron ferritic in terms of area occupation ratio.

Description

Electric resistance welded steel pipe

technical field

The present invention relates to an electric resistance welded steel pipe which is excellent in low-temperature toughness and has a low yield ratio, which is most suitable for applications such as line pipes for transportation of oil and natural gas.

Background technique

In pipelines that transport oil and gas over long distances, the improvement of transmission efficiency due to high pressure and the laying of pipelines to deep seas are advancing. For this reason, electric resistance welded steel pipes used in pipelines are required to be thicker and stronger. In addition, when the pipeline is laid in the deep sea, the electric resistance welded steel pipe will be bent and bent under load, so a low yield ratio is required so that buckling does not occur.

When the wall thickness of the electric resistance welded steel pipe becomes thicker, the processing strain introduced when the electric resistance welded steel pipe is produced from the hot-rolled steel sheet becomes larger. Therefore, it becomes difficult to suppress an increase in the yield ratio. The yield ratio is an indicator of the durability of the material until a stress greater than the yield stress is added to the material until it yields to buckling or fracture. The lower the yield ratio, the harder it is for the steel pipe to buckle. The yield ratio (hereinafter also referred to as "YR") is a value represented by the ratio (YS/TS) of yield stress (hereinafter also referred to as "YS") to tensile strength (hereinafter also referred to as "TS").

Generally, it is known that YR decreases when the metal structure of steel materials is multiphase structure including soft phase and hard phase, and an electric resistance welded steel pipe in which the metal structure of the base material is multiphase structure has been proposed.

Patent Document 1 discloses a low yield ratio electric resistance welded steel pipe in which island martensite and retained austenite are formed as the second phase. Patent Document 2 discloses a low-yield-ratio hot-rolled steel sheet to be a raw material for line pipe produced by spiral pipemaking and UO pipemaking.

prior art literature

Patent Document 1: Japanese Patent Application Laid-Open No. 5-105952

Patent Document 2: Japanese Patent Application Laid-Open No. 10-176239

Contents of the invention

If the base material of the electric resistance welded steel pipe becomes thicker and the outer diameter becomes smaller, the processing strain introduced when forming the steel sheet or strip into a tubular shape will increase, making it difficult to maintain a low yield ratio after pipe making. Especially when the strength level is X60 according to the American Petroleum Institute (API) standard (tensile strength 520MPa or more), the ratio t/D of the wall thickness t to the outer diameter D is 5% or more. In the case of manufacturing, it is difficult to maintain the yield ratio at 90% or less.

In addition, in order to lower the yield ratio, it is necessary to form a multiphase structure including soft phases and hard phases, and it is difficult to ensure low-temperature toughness in a multiphase structure including ferrite and martensite. However, for electric resistance welded steel pipes used in pipelines, excellent toughness is required together with low yield ratio, and electric resistance welded steel pipes having these characteristics are required.

The present invention has been made in view of these actual circumstances, and provides a thick-walled electric resistance welded steel pipe capable of maintaining a low yield ratio while maintaining the pipe-making state, and a manufacturing method thereof.

In conventional electric resistance welded steel pipes having a multiphase structure, Nb is added to precipitate NbC in ferrite to secure strength. However, as a result of studies by the present inventors, it was found that adding a large amount of Nb increases the yield stress of a hot-rolled steel sheet as a steel pipe material, and as a result, it is difficult to achieve a lower yield ratio after pipemaking. Therefore, the inventors of the present invention have studied to achieve higher strength and lower yield ratio by the hard phase of the second phase rather than by precipitation strengthening.

In two-phase steel, dislocations are introduced into the soft phase around the hard phase during plastic deformation to work harden. Therefore, if the deformation of the hard phase is suppressed, accumulation of dislocations in the soft phase is promoted, and the work hardening rate can be increased. Therefore, in ferrite-martensite dual-phase steel, the harder the martensite (hard phase) as the second phase, the higher the work hardening rate of ferrite, and the work hardening properties of steel sheets and steel pipes improve.

After the steel is hot-rolled, it is cooled to room temperature at an accelerated rate, whereby pearlite transformation and bainite transformation can be suppressed, and hard martensite (hard phase) can be formed. On the other hand, after cooling, if retained austenite is contained in the hard phase without transforming into martensite, the work hardening properties will decrease.

Therefore, the inventors of the present invention focused on suppressing the amount of Nb added as described above and also reducing the amount of C to form a heterogeneous structure that suppresses the formation of retained austenite. As a result, they found that strength, and can obtain ferrite-martensitic dual-phase steel with low yield ratio.

In addition, the inventors of the present invention conducted detailed studies on the yield ratio influenced by the hard phase of the second phase. As a result, it was found that by setting the cooling after hot rolling as a two-stage cooling in which the cooling rate is changed at 650° C., and by setting the coiling temperature after hot rolling at a low temperature, it is possible to achieve refinement of the hard phase and a hard phase. can reduce the yield ratio.

In addition, as a result of studies conducted by the present inventors to obtain good toughness together with the above-mentioned high strength and low yield ratio, it was found that by controlling the hot rolling conditions, the ferrite grain size becomes finer, and as a result, The deterioration of the toughness of the steel pipe can be suppressed by making the hard phase transformation after coiling finer.

Based on the above findings, the present inventors have accomplished the present invention. Its gist is as follows.

(1) An electric resistance welded steel pipe, characterized in that the composition of the base metal is in mass %, containing

C: 0.05～0.10%,

Mn: 1.00～1.60%,

Ti: 0.005～0.030%,

Nb: 0.005% or more and less than 0.035%, and

N: 0.001～0.008%

also contains

Si: 0.01 to 0.60%, and

Al: 0.001～0.10%

one or both of them, and limited to

P: less than 0.02%,

S: 0.005% or less, and

The balance is iron and unavoidable impurities,

Ceq represented by the following (Formula 1) satisfies 0.23≤Ceq≤0.38, and,

The metal structure of the base material contains 3 to 13% of martensite in terms of area ratio, and the balance is ferrite.

Ceq＝C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15...(Formula 1)

Here, C, Mn, Cr, Mo, V, Ni, and Cu in (Formula 1) are values showing the content of each element in mass %.

(2) The electric resistance welded steel pipe according to (1) above, wherein the composition of the base material is expressed in mass %, and further contains

Ni: 1.0% or less,

Cu: 1.0% or less,

Cr: 1.0% or less,

Mo: 0.5% or less,

V: 0.2% or less,

Ca: 0.006% or less, and

REM: less than 0.006%

1 or more of them.

According to the electric resistance welded steel pipe described in the above (1), it is characterized in that the composition of the base material satisfies

Mn: 1.00～1.50%,

Si: 0.40% or less,

Also satisfy: 0.23≤Ceq≤0.30,

The average value of the circle-equivalent particle size of martensite in the metallic structure of the base material is 0.5 to 1.5 μm, and,

The tensile strength of the steel pipe is 520-790MPa.

(4) The electric resistance welded steel pipe according to the above (3), wherein the composition of the base material satisfies

Nb: 0.005 to 0.020%.

(5) The electric resistance welded steel pipe according to the above (3) or (4), characterized in that the composition of the base material is expressed in mass%, and further contains

Ni: 0.5% or less,

Cu: 0.5% or less,

Cr: 0.5% or less,

Mo: 0.2% or less,

V: 0.1% or less,

Ca: 0.006% or less, and

REM: less than 0.006%

1 or more of them.

According to the present invention, the strength level can be provided according to the American Petroleum Institute (API) standard as X60-X70 (the tensile strength of the steel pipe is 520-790 MPa), has sufficient low-temperature toughness, and even under the condition of maintaining the pipe-making state, the yield ratio It can also be an electric resistance welded steel pipe of 90% or less and its manufacturing method.

Description of drawings

FIG. 1 is a graph illustrating the relationship between the area ratio of martensite and the yield ratio.

FIG. 2( a ) is an optical micrograph of a conventional high-Nb and high-C electric resistance welded steel pipe, and FIG. 2( b ) is an optical micrograph observed after it has been subjected to LePera etching.

FIG. 3( a ) is an optical micrograph of an electric resistance welded steel pipe having a composition within the range of the present invention, and FIG. 3( b ) is an optical micrograph of the steel pipe subjected to LePera etching.

detailed description

In order to reduce the yield ratio of the electric resistance welded steel pipe, it is important to form a multiphase structure including a soft phase and a hard phase in the metal structure of the hot-rolled steel sheet serving as the base material. In the present invention, the soft phase is defined as ferrite, and the hard phase is defined as martensite. Furthermore, the yield ratio can be lowered by lowering the coiling temperature in hot rolling.

2 and 3 show the observation results of martensite in the conventional electric resistance welded steel pipe and the electric resistance welded steel pipe of the present invention. When LePera etching is performed, martensite can be observed as a whitened phase with an optical microscope. Therefore, the area ratio of martensite can be obtained from the microstructure photograph.

Fig. 2(a) is an optical micrograph of a conventional high-Nb and high-C electric resistance welded steel pipe to which excessive amounts of Nb and C have been added, and Fig. 2(b) is an optical micrograph observed after LePera etching.

Fig. 3(a) is an optical microscope photograph of an electric resistance welded steel pipe having a composition within the scope of the present invention, and Fig. 3(b) is an optical microscope photograph observed after LePera etching thereof.

As can be seen from a comparison of Fig. 2(b) and Fig. 3(b), in the case of conventional electric resistance welded steel pipes that utilize precipitates such as NbC to achieve high strength, the phase that is whitened by LePera etching, that is, martensite Basically not observed, but martensite was observed in the case of the present invention of Fig. 3(b).

In addition, retained austenite was also observed as a whitened phase in LePera etching, so the volume ratio of retained austenite in FIG. 3( b ) was measured by X-ray diffraction. As a result, the volume fraction of retained austenite was 1% or less. If the volume fraction of retained austenite is 1% or less, it will not affect the characteristics of the electric resistance welded steel pipe of the present invention.

Tensile test is carried out on the yield ratio, and YS/TS is obtained, expressed as a percentage. The results of examining the relationship between the area ratio of martensite and the yield ratio are shown in FIG. 1 . As shown in FIG. 1 , when the area ratio of martensite becomes 3% or more, the yield ratio becomes 90% or less. Furthermore, when the area ratio of martensite becomes 8% or more, the yield ratio drops rapidly, and the yield ratio can be lowered to 80% or less.

Hereinafter, the electric resistance welded steel pipe of the present invention and its manufacturing method will be described in detail.

First, the base material components of the electric resistance welded steel pipe of the present invention will be described. The composition of the hot-rolled steel sheet as the material of the electric resistance welded steel pipe is the same as that of the base material of the electric resistance welded steel pipe. The following "%" means "mass %".

<C:0.05～0.10%>

C is an element useful for increasing the strength of steel. It increases martensite to harden the steel and also contributes to the reduction of the yield ratio, so the lower limit is made 0.05%. If the amount of C exceeds 0.10%, on-site weldability deteriorates, and the area ratio of martensite increases, strength becomes too high, and toughness deteriorates, so the upper limit is made 0.10%. From the viewpoint of securing the strength, the amount of C is preferably 0.06% or more. From the viewpoint of ensuring toughness without excessively increasing the strength, the amount of C is preferably 0.08% or less.

<Mn: 1.00～1.60%>

Mn is an element that improves the hardenability of steel and contributes to the formation of martensite. In the present invention, 1.00% or more of Mn is added to ensure strength. If Mn is added excessively, the area ratio of martensite increases and the toughness deteriorates, so the upper limit is made 1.60%. From the viewpoint of ensuring strength, the amount of Mn is preferably 1.10% or more, more preferably 1.20% or more. From the viewpoint of ensuring toughness, the amount of Mn is preferably 1.50% or less, more preferably 1.40% or less.

<Ti: 0.005～0.030%>

Ti is an element that forms carbonitrides, refines the structure, and contributes to the improvement of toughness. The invention of the present application uses a thick-walled steel plate, and especially in order to ensure the toughness of the thick-walled steel plate at low temperature, it is necessary to add 0.005% or more of Ti. If Ti is added excessively, coarsening of TiN and precipitation hardening by TiC will occur, the toughness will deteriorate, and the yield ratio will increase, so 0.030% is made the upper limit. From the viewpoint of making the structure finer and ensuring toughness, the amount of Ti is preferably 0.008% or more, more preferably 0.010% or more. From the viewpoint of suppressing a drop in yield ratio due to precipitates, the amount of Ti is preferably 0.025% or less, more preferably 0.020% or less.

<Nb: 0.005% or more and less than 0.035%>

Nb is an element that lowers the recrystallization temperature, and suppresses the recrystallization of austenite during hot rolling and contributes to the refinement of the structure, so it is added in an amount of 0.005% or more. If Nb is added excessively, the yield stress will increase due to excessive precipitation strengthening and the yield ratio will increase, so the content is less than 0.035%. From the viewpoint of reducing the yield ratio, the amount of Nb is more preferably 0.025% or less, further preferably 0.020% or less.

<N: 0.001～0.008%>

N is an element that forms nitrides, especially TiN, and contributes to the micronization of the structure, so it is contained in an amount of 0.001% or more. In order to make the structure finer, it is preferable to contain 0.002% or more of N, and it is more preferable to make the content 0.003% or more. If the amount of N becomes excessive, coarse nitrides will be generated and the toughness will be impaired, so the upper limit is made 0.008%. The upper limit of the amount of N is preferably 0.007%, more preferably 0.006%.

In the present invention, one or two of Si and Al are used as deoxidizing elements.

<Si: 0.60% or less>

Si is effective as a deoxidizer. When Al is added, it is not necessarily added. In order to obtain the effect as a deoxidizer, it is preferable to add 0.01% or more. In addition, Si is an element that increases strength by solid solution strengthening, so it is more preferably added in an amount of 0.05% or more, and more preferably added in an amount of 0.10% or more. If Si addition exceeds 0.60%, ductility and toughness will be impaired, and furthermore, electric weldability will be impaired, so the upper limit is made 0.60%. From the viewpoint of ensuring toughness, the amount of Si is preferably 0.40% or less, more preferably 0.30% or less.

<Al: 0.10% or less>

Al is effective as a deoxidizer. When Si is added, it is not necessarily added. In order to obtain the effect as a deoxidizer, it is preferable to add 0.001% or more. In order to enhance the effect of deoxidation, it is preferable to add 0.005% or more of Al, and it is more preferable to add 0.01% or more of Al. If Al addition exceeds 0.10%, inclusions increase and ductility and toughness are impaired, so it is limited to 0.10% or less. From the viewpoint of ensuring toughness, the amount of Al is preferably 0.05% or less, more preferably 0.03% or less.

In the present invention, the content of P and S as impurities is limited. P and S are not intentionally added elements, but are mixed with P and S contained in raw materials. However, it is not good if the content is too large, so the restrictions are as follows.

<P: 0.02% or less>

P is an impurity, and the upper limit of the content is made 0.02%. By reducing the amount of P, grain boundary fracture is prevented and toughness is improved, so the amount of P is preferably 0.015% or less, more preferably 0.010% or less. The amount of P is preferably small, so no lower limit is set. From the viewpoint of the balance between characteristics and cost, it usually contains 0.001% or more.

<S: 0.005% or less>

S is an impurity, and the upper limit of the content is made 0.005%. By reducing the amount of S, MnS elongated by hot rolling can be reduced to improve toughness, so the amount of S is preferably 0.003% or less, more preferably 0.002% or less. The amount of S is preferably small, so no lower limit is set. In view of the balance between characteristics and cost, it is usually contained at 0.0001% or more.

<Ceq: 0.23～0.38>

The carbon equivalent Ceq is an indicator of hardenability and is sometimes used as an indicator of strength. From the content [mass %] of C, Mn, Cr, Mo, V, Ni, and Cu, it can be obtained by the following (Formula 1). To secure the strength, it is necessary to set Ceq to 0.23 or more. To ensure toughness, it is necessary to set Ceq to 0.38 or less. The lower limit of Ceq is preferably 0.25 or more. The upper limit of Ceq is preferably 0.35 or less, more preferably 0.30 or less.

Ceq＝C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15...(Formula 1)

Here, C, Mn, Cr, Mo, V, Ni, and Cu are the content [% by mass] of each element. In addition, Cr, Mo, V, Ni, and Cu are elements that are selectively added in the present invention as described later, and are calculated as 0 in the above-mentioned (Formula 1) when they are not intentionally added.

In the present invention, one or two or more of Ni, Cu, Cr, Mo, and V may be added in order to improve the hardenability and strength of the steel. In addition, in order to improve the toughness of steel pipes and electric resistance welded parts, one or two or more of Ca and REM may be added. These elements are arbitrarily added elements and are not necessarily added elements, so the lower limit of the content is not specified. The preferable lower limit value is described in the following description, but this is the preferable lower limit value for obtaining the effect of improving the hardenability and improving the strength by adding each element. Even if the content of each element is lower than the preferable lower limit, it does not exert a bad influence on steel.

<Ni: 1.0% or less>

Ni is an element that improves the hardenability of steel and also contributes to the improvement of toughness. In order to improve the strength, the amount of Ni is preferably 0.05% or more. Moreover, since Ni is an expensive element, the upper limit is made into 1.0%, More preferably, it is 0.5% or less, More preferably, it is 0.3% or less.

<Cu: 1.0% or less>

Cu is an element that improves the hardenability of steel and also contributes to solid solution strengthening, so it is preferably added in an amount of 0.05% or more. Excessive addition of Cu will impair the surface properties of the steel sheet, so the upper limit is made 1.0% or less. From the viewpoint of economic efficiency, the more preferable upper limit of the amount of Cu is 0.5%, and it is still more preferably 0.3% or less. When Cu is added, it is preferable to add Ni simultaneously from the viewpoint of preventing deterioration of surface properties.

<Cr: 1.0% or less>

Cr is an element effective in improving strength, and it is preferable to add 0.05% or more. If Cr is added excessively, weldability may deteriorate when butt-welding (on-site welding) the ends of the steel pipes, so 1.0% is made the upper limit. More preferably, it is 0.5% or less, and still more preferably, it is 0.2% or less.

<Mo: 0.50% or less>

Mo is an element that contributes to high strength of steel, and it is preferable to add 0.05% or more. However, Mo is an expensive element, and its upper limit is 0.5%. A more preferable upper limit of the amount of Mo is 0.3% or less, still more preferably 0.1% or less.

<V: 0.2% or less>

V is an element that forms carbides and nitrides and increases the strength of steel by precipitation strengthening. In order to effectively increase the strength, it is preferable to add 0.01% or more. If V is added excessively, carbides and nitrides may coarsen and the yield ratio may increase, so the upper limit of the amount of V is made 0.2%. From the viewpoint of reducing the yield ratio, the upper limit of the amount of V is more preferably 0.1% or less, more preferably 0.05% or less.

<Ca: 0.006% or less><REM: 0.006% or less>

Ca and REM control the form of sulfide-based inclusions to improve low-temperature toughness, and further refine the oxides in the resistance-welded zone to improve the toughness of the resistance-welded zone. Therefore, it is preferable to add one or both of them at 0.001% or more. If Ca and REM are added excessively, oxides and sulfides will increase in size and adversely affect toughness, so the upper limit of the added amount is 0.006%. Here, REM is a generic term for Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

In the component composition of the base material of the electric resistance welded steel pipe according to the present invention, the balance other than those described above is iron and unavoidable impurities. The so-called unavoidable impurities are components contained in raw materials or mixed in the manufacturing process, and are called components not intentionally contained in steel.

Specifically, P, S, O, Sb, Sn, W, Co, As, Mg, Pb, Bi, B, and H are exemplified. However, P and S need to be controlled to be 0.02% or less and 0.005% or less, respectively, as described above. O is preferably controlled to be 0.006% or less.

For other elements, it is usually possible to mix in unavoidable impurities such as Sb, Sn, W, Co, and As at 0.1% or less, Mg, Pb, and Bi at 0.005% or less, and B and H at 0.0005% or less. range, no special control is required.

In addition, the essential selection or optional additive elements in the steel pipe of the present invention, that is, Si, Al, Ni, Cu, Cr, Mo, V, Ca, and REM, may be mixed as unavoidable impurities even if they are not intentionally contained, As long as it is below the upper limit of the above-mentioned intentionally contained content, it will not have a bad influence on the steel pipe of the present invention, so there is no problem. In addition, N is generally treated as an unavoidable impurity in steel, but in the electric resistance welded steel pipe of the present invention, it needs to be controlled within a certain range as described above.

Next, the metal structure of the base material of the electric resistance welded steel pipe of the present invention will be described.

The metal structure of the base material of the electric resistance welded steel pipe of the present invention is a multiphase structure in which martensite and the remainder contain ferrite. In order to reduce the yield ratio, the area ratio of martensite is set to 3% or more. The area ratio of martensite is preferably 5% or more, more preferably 8% or more. Since the toughness decreases when martensite increases, the upper limit of the area ratio of martensite is made 13%. The area ratio of martensite is preferably 12% or less, more preferably 10% or less.

The area ratio of martensite was obtained by performing LePera etching with an optical microscope. When retained austenite increases, the hardness of martensite decreases and the yield ratio increases. Therefore, in the present invention, the volume ratio of retained austenite is measured by the X-ray diffraction method, and if the volume ratio of retained austenite is 1% or less, it is judged that the metal structure is a multi-layer structure including martensite and ferrite. phase organization.

Martensite is preferably dispersed in ferrite so that the average equivalent circle particle size is 0.5 to 1.5 μm. If the average value of the circle-equivalent particle size of martensite is less than 0.5 μm, it does not contribute to a decrease in the yield ratio. If the average value of the circle-equivalent particle size of martensite exceeds 1.5 μm, the low-temperature toughness will be impaired. It is more preferable that the average value of the circle-equivalent grain size of martensite is 0.7 to 1.1 μm. In a more preferable dispersed state of martensite, the maximum value of the equivalent circle particle size is 7 μm or less, more preferably 5 μm or less, and the standard deviation is 1 μm or less, more preferably 0.8 μm or less.

Next, the method of manufacturing the electric resistance welded steel pipe of the present invention will be described.

First, the production conditions of the hot-rolled steel sheet as the electric resistance welded steel pipe material of the present invention will be described.

In the present invention, the steel having the above composition is smelted, cast to form a steel sheet, heated and hot-rolled, cooled under controlled conditions, coiled and air-cooled to produce a hot-rolled steel sheet.

The heating temperature of the steel sheet is preferably 1150° C. or higher in order to form a solid solution of carbide-forming elements such as Nb in the steel. If the heating temperature is too high, the structure will become coarse, so in order to prevent the grain size of ferrite from coarsening, it is preferably 1250° C. or lower.

Hot rolling needs to be performed in a temperature range where the structure of the steel is in the austenite phase. This is because if rolling is performed after ferrite transformation starts, processed ferrite is formed and the anisotropy of properties increases. Therefore, the completion temperature of hot rolling is preferably Ar ₃ or higher at which ferrite transformation starts during cooling. If the finish temperature is too high, the structure will become coarse, so it is preferably Ar ₃ +50°C or lower.

Ar ₃ can be obtained from the thermal expansion behavior during heating and cooling using a test material with the same composition as the base steel plate. In addition, it can also be obtained from the composition of the base steel sheet by the following (Formula 2).

Ar ₃ (℃)＝910-310C-80Mn-55Ni-20Cu

-15Cr-80Mo...(Equation 2)

Here, C, Mn, Ni, Cu, Cr, and Mo are the content [% by mass] of each element. Ni, Cu, Cr, and Mo are optional additive elements in the present invention. When these elements are not intentionally added, they are calculated as 0 in the above (Formula 2).

In hot rolling, in order to make the ferrite structure of steel finer, it is preferable to set the rolling reduction at 950° C. or lower to 70% or more. Depending on the thickness of the steel sheet, hot rolling may be performed at a temperature exceeding 950°C, but in order to promote ferrite transformation, it is preferable to increase the rolling reduction at 950°C or lower to accumulate strain. The rolling reduction below 950°C is calculated as a percentage by dividing the difference between the plate thickness at 950°C and the plate thickness after finish rolling by the plate thickness after finish rolling.

After hot rolling, accelerated cooling is performed from a temperature of 750° C. or higher, preferably Ar ₃ point or higher, in order to form martensite. After hot rolling, if the temperature drops too much, coarse polygonal ferrite is formed, the strength is lowered, and the toughness is deteriorated.

Accelerated cooling is set to two-stage cooling, the average cooling rate of the first stage up to 650°C is set to 10-25°C/s, and the average cooling rate of the latter stage until the accelerated cooling stops below 650°C is set to 20-50°C/s s. The cooling rate in the rear stage is 1.5 times or more, preferably 2 times or more, the cooling rate in the front stage.

As mentioned above, the reason why the two-stage cooling is used is that ferrite is formed by cooling in the first stage, and the cooling rate is increased in the latter stage, so that pearlite is not formed, and austenite is not left, and martensite is obtained. multiphase structure of ferrite and martensite.

The stop temperature of accelerated cooling is 300° C. or lower which is sufficiently lower than the Ms point, and by coiling to produce a hot-rolled steel strip, hard martensite can be formed and the yield ratio can be lowered. If the accelerated cooling stop temperature exceeds 100° C., the area ratio of martensite is insufficient, excessive austenite remains, and the yield ratio does not sufficiently decrease.

Next, in the present invention, the obtained hot-rolled steel strip is air-cooled, formed into a tubular shape in the cold state, and the ends are butted together to perform resistance welding to manufacture an electric resistance welded steel pipe. The present invention sets an electric resistance welded steel pipe having a thick wall and a small outer diameter. Although not specifically specified, the ratio t/D of the wall thickness t of the base material to the outer diameter D of the electric resistance welded steel pipe is about 2.0 to 6.0%, which can meet the requirements of pipelines laid in deep sea. Electric resistance welded steel pipes where /D is 5.0% or more.

In addition, only the electric resistance welded part may be heated, and the seam heat treatment of accelerated cooling may be performed. In resistance welding, the butt joint is heated to melt, and a pressure is applied to perform solid-phase bonding, whereby the vicinity of the resistance weld is plastically deformed at a high temperature, and then rapidly cooled. Therefore, the electric resistance welded portion is hardened compared with the base material, and by performing seam heat treatment, the low temperature toughness and deformability of the electric resistance welded steel pipe can be further improved.

Example

Hereinafter, the effects of the present invention will be more specifically described through examples. Furthermore, the present invention is not limited to the conditions used in the following examples. In addition, the underline in Tables 1-3 shows that it is out of the range of this invention. In addition, an empty column in the table indicates that the element was not intentionally added. Steels AA to AG are steels that do not satisfy the composition requirements of the present invention.

Steels having the chemical compositions shown in Table 1 were cast to form steel sheets. These steel sheets were heated to the heating temperature shown in Table 2, and the rolling completion temperature (FT in Table 2) was set to Ar ₃ point or higher, hot rolling was performed, and accelerated cooling was carried out to obtain base material steel sheets. Accelerated cooling is a two-stage cooling in which the cooling rate is changed at a limit of 650°C, and the average cooling rate in the latter stage (below 650°C) is about twice the average cooling rate in the former stage (up to 650°C). The accelerated cooled steel sheet was coiled at the coiling temperature (CT) described in Table 2 to form a hot rolled strip.

Next, after the obtained hot-rolled steel strip is air-cooled, it is formed into a tubular shape by a continuous roll forming process, and the ends of the hot-rolled steel strip are butt-jointed for resistance welding. Thereafter, after heating the resistance welded portion as necessary, accelerated cooling was performed to perform seam heat treatment.

The "reduction amount" in Table 2 is the reduction amount below 950°C in the hot rolling process, and the difference between the plate thickness at 950°C and the plate thickness after finish rolling is divided by the plate thickness after finish rolling, Obtained as a percentage. In addition, "t" represents the thickness of the steel plate, and "D" represents the outer diameter of the steel pipe after pipe making.

Ar ₃ in Table 1 was obtained from the contents [mass %] of C, Mn, Ni, Cu, Cr, and Mo shown in Table 1. In addition, Ni, Cu, Cr, and Mo are optional additive elements in the present invention, and as shown in the blank column of Table 1, when they are not intentionally added, they are calculated as 0 in the following (Formula 2) .

Ar ₃ (℃)＝910-310C-80Mn-55Ni-20Cu

-15Cr-80Mo···(Formula 2)

Next, a sample for microstructure observation was prepared from the base material portion of the obtained electric resistance welded steel pipe, and a section parallel to the longitudinal direction of the steel pipe was etched with nitric acid alcohol, and the microstructure was observed and photographed with an optical microscope. The observation position was set at t/4 position from the outer surface. Using these structure photographs, it was confirmed that structures other than ferrite and martensite such as pearlite and bainite were not formed. Thereafter, LePera etching was performed, an optical microscope photograph was taken at a position of 0.4t, and the area ratio of martensite was obtained by image analysis. In addition, the circle-equivalent particle size of martensite was measured by image analysis. Regarding the area ratio of martensite and the equivalent circle radius, 10 viewing fields of 100 μm×200 μm were measured, and an average value was obtained. In addition, the volume fraction of austenite was measured by X-ray diffractometry and confirmed to be 1% or less.

Next, according to JIS Z 2241, from the base material of the electric resistance welded steel pipe, an arc-shaped tensile test piece was prepared along the length direction of the steel pipe, and the tensile test was carried out at room temperature to obtain the yield stress and tensile strength. In addition, V-notch test pieces were prepared from base materials of electric resistance welded steel pipes in accordance with JIS Z 2242, and Charpy tests were performed at -30°C to obtain Charpy absorbed energy vE _-30 and evaluate toughness. Furthermore, the V-notch test piece is produced with the circumferential direction as the longitudinal direction. In the case where a full-size test piece with a thickness of 10mm cannot be obtained, an auxiliary size test piece is formed, which is converted to a thickness of 10mm. The results are shown in Table 3.

As shown in Table 3, the examples of the present invention all include the metal structure of martensite and ferrite with an appropriate area ratio, the tensile strength of the electric resistance welded steel pipe is all above X56 (more than 490MPa in tensile strength), and the yield ratio is all 90% or less, good. In addition, the examples of the present invention showed high Charpy absorbed energy of 190J or more even at -30°C, and had good toughness.

In No. 21, since the amount of C is small, the strength is lowered, and the formation of martensite becomes insufficient, which is an example in which the yield ratio increases. No. 22 has a large amount of C, and No. 23 has a large amount of Mn, and is an example in which martensite is excessively formed and toughness is lowered. Since No. 24 has a small amount of Mn, it is an example of a decrease in strength.

Ceq of No. 25 is too high, and it is an example in which martensite is excessively formed and toughness falls. Ceq of No. 26 is too low, which is an example of strength drop. In No. 27, since the amount of Ti was small, the toughness was lowered, and since the amount of Nb was large, bainite was formed in addition to ferrite, and the yield ratio was increased.

No. 28 has a slow cooling rate below 650°C, so martensite is not formed, and the yield ratio is increased. On the other hand, No. 29 is an example in which the accelerated cooling rate is as high as 450°C, martensite is not formed, and the yield ratio increases.

Industrial Utilization Possibility

According to the present invention, it is possible to provide an electric resistance welded steel pipe having a strength of X60 to 70 grades used for pipelines laid in deep sea and the like, having sufficient low temperature toughness, and a low yield ratio, and thus has great industrial applicability.

Claims (5)

1. An electric resistance welded steel pipe, characterized in that the composition of the base metal is in mass %, containing C: 0.05～0.10%, Mn: 1.00～1.60%, Ti: 0.005～0.030%, Nb: 0.005% or more and less than 0.035% and N: 0.001～0.008%, also contains Si: 0.01 to 0.60% and Al: 0.001～0.10% one or both of them, and limited to P: less than 0.02%, S: 0.005% or less, The balance is iron and unavoidable impurities, Ceq represented by the following (Formula 1) satisfies 0.23≤Ceq≤0.38, and, The metal structure of the base metal contains 3 to 13% martensite in terms of area ratio, and the balance is ferrite. The Charpy absorbed energy at -30°C is above 190J, Ceq＝C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15...(Formula 1) Here, C, Mn, Cr, Mo, V, Ni, and Cu in (Formula 1) are values showing the content of each element in mass %. 2. The electric resistance welded steel pipe according to claim 1, characterized in that, the composition of the base material is expressed in mass%, and further contains Ni: 1.0% or less, Cu: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, V: 0.2% or less, Ca: 0.006% or less and REM: less than 0.006% 1 or more of them. 3. The electric resistance welded steel pipe according to claim 1, wherein the composition of the base material satisfies Mn: 1.00～1.50%, Si: 0.40% or less, Also satisfy: 0.23≤Ceq≤0.30, The average value of the circle-equivalent particle size of martensite in the metallic structure of the base material is 0.5 to 1.5 μm, and, The tensile strength of the steel pipe is 490-760MPa. 4. The electric resistance welded steel pipe according to claim 3, wherein the composition of the base material satisfies Nb: 0.005 to 0.020%. 5. The electric resistance welded steel pipe according to claim 3 or 4, characterized in that, the composition of the base material is expressed in mass%, and further contains Ni: 0.5% or less, Cu: 0.5% or less, Cr: 0.5% or less, Mo: 0.2% or less, V: 0.1% or less, Ca: 0.006% or less and REM: less than 0.006% 1 or more of them.